1. Field of the Invention
The present invention relates to a control device for a vehicle a.c. generator, in particular to the arrangement of a field current control circuit for controlling a field current in an a.c. generator.
2. Discussion of Background
As a control device for a vehicle a.c. generator, there has been known the one shown in FIG. 4. In this Figure, reference numeral 1 designates an a.c. generator which includes armature coils 101 and a field winding 102. Reference numeral 2 designates a rectifier which includes an output terminal 201 and a ground (earth) terminal 202. Reference numeral 3 designates a field current control circuit which is constituted by a semiconductor switching element 301 and a suppression diode 302, and which includes four terminals of a power source terminal "a", a field current control terminal "b", a control signal input terminal "c" and a ground (earth) terminal "d". Reference numeral 4 designates a control unit which is mounted on a vehicle and includes a microcomputer. Reference numeral 5 designates a battery, and reference numeral 6 designates a key switch.
Now, the operation of the control device will be explained. When a control signal is sent out from the control unit 4 to the control signal input terminal "c" of the field current control circuit 3, the semiconductor switching element 301 is driven to conduction to flow a field current in the field winding 102, generating electricity at the a.c. generator 1 The output from the a.c. generator is rectified by the rectifier 2, and charges the battery 5. On the other hand, the control unit 4 detects the terminal voltage of the battery 5. When the control unit detects that the terminal voltage of the battery becomes higher than a predetermined voltage, the control unit cuts off the control signal to the field current control circuit 3 to decrease the field current so as to lower the output from the a.c. generator, stopping charging the battery. Such a operation is repeated to control the output voltage of the a.c. generator.
The field current control circuit 3 for that type of control device is arranged separately from the output portion of the a.c. generator 1 (the rectifier 2 in the shown example) to optimize temperature characteristics as disclosed in JP-U-54-140224. The field current control circuit is arranged as shown in FIG. 5. Specifically, the semiconductor switching element (e.g. transistors) 301 has the collector connected to the field current control terminal "b", the base connected to the control signal input terminal "c", and the emitter connected to the ground terminal "d", respectively. The suppression diode 302 is connected between the power source terminal "a" and the field current control terminal "b".
Since the control device stated above is constructed with the field current control circuit 3 having the power source terminal "a" adjoining the control signal input terminal "c", the control device has created problems in that the power source terminal "a" and the control signal input terminal "c" are readily short-circuited by entrance of water, for example, and that in such case a short-circuit current flows from the battery 5 directly into the control signal input terminal "c", the semiconductor switching element 301 is driven to conduction to increase the field current, and the output voltage from the a.c. generator is extraordinarily raised to overcharge the battery 5 or damage an electrical load such as a headlight due to overvoltage, and further to damage an electrical system for ignition or the control unit, thereby stopping the engine.